Contact spring and socket combination for high bandwidth probe tips

ABSTRACT

A high-bandwidth electrical test probe having a probe contact spring of reduced size and characteristic capacitance is presented. The probe includes a contact spring connected at one end to the input port of a probe circuit. The opposite end of the contact spring enters the a probe socket and a predetermined angle of entry. The probe socket has a bore formed therein which is arranged at a non-zero angle relative to the angle of entry of the contact spring into said probe socket bore, thereby guaranteeing electrical contact with the bore. The design allows the use of a very small contact spring, on the order of tens of mils, thereby reducing the parasitic capacitance of the spring and allowing much higher bandwidths than heretofore achievable.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 10/100,677 filed on Mar.18, 2002, now U.S. Pat. No. 6,911,811 the entire disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention pertains generally to electronic testinstrumentation, and, more particularly, to a spring and socket assemblyfor a high bandwidth electronic test probe.

The increasing reliance upon computer systems to collect, process, andanalyze data has led to the continuous improvement of the systemcomponents and associated hardware. New methods for increasing the speedof integrated circuit components while also increasing the functionaldensity and reducing the physical size of integrated circuits areconstantly being sought. As a result, it is not uncommon to seeintegrated circuits running at several GHz with pin spacing on the orderof 10 mil apart.

In a test environment, electronic test instruments such as oscilloscopesand logic analyzers are often required to measure electrical parameterson device pins or nodes of a circuit. A common tool for collectingmeasurements in this environment is an electrical test probe. Anelectrical test probe is used to make a connection between a test pointor signal source on a device/circuit under test and a test instrument.An electrical test probe comprises a cable having a connector at one endconnectable to the electronic instrument and having a contact devicesuch as a probe pin at the other end of the cable for probing the testpoint (e.g., a desired device pin or circuit node). Typically, thecontact device includes a probe pin connected to probe circuitry whichfilters a signal seen on the probe pin. The probe pin may be manuallyspringably connectable to the probe circuitry via a spring mechanism.

As the speed of integrated circuits increase, the bandwidth required ofelectrical test probes has exceeded that which can be achieved withprior art probes. As a general rule, in order to achieve accuratemeasurements, the bandwidth of a test probe should be approximately fivetimes greater than the frequency of the waveform being measured.

FIG. 10 is a top view and FIG. 11 is a cross-sectional side view of aprior art electrical test probe tip 20. As shown, test probe tip 20includes circuitry implemented on a printed circuit board 22. Theprinted circuit board 22 includes an input port 23 for receiving signalsfrom a contact spring 25, and an output port 24 for electricalconnection to a probe cable 21.

The printed circuit board 22 and probe pin 26 are positioned within ahousing 28. In order to achieve maximum electrical contact, prior artcontact spring mechanisms 25 were formed as a flat piece of metal withwidth d shaped into a hook, as illustrated in FIGS. 11 and 12. The widthd of such prior art hooks is typically on the order of approximately100-200 mils wide. Due to the large width d of the contact spring 25,the contact spring 25 exhibits a large parasitic capacitance C_(hook)which prevents signals above a certain cutoff frequency f_(o) frompassing. The cutoff frequency of the contact spring 25 is the frequencyof the wave when the wavelength λ is twice the width d of the contactspring 25. At this frequency, λ/2 resonances occur that cause thecontact spring 25 to act inductively. Above the cutoff frequency,additional resonances occur regularly. Therefore, the cutoff frequencyrepresents the upper limit of the capacitor's (i.e., contact spring 25)frequency range. As is known in the art, the larger the width d of thecontact spring, the greater its parasitic capacitance and inductance andtherefore the lower the cutoff frequency of the probe.

Accordingly, there exists a need in the industry for a high bandwidthelectrical test probe. In particular, a need exists for a probe contactspring of much smaller size and therefore reduced characteristiccapacitance that also ensures good electrical contact.

In addition, as the node size and the spacing between nodes is reduced,the size of the probe tips must also accordingly be decreased in orderto accommodate the required spacing between the nodes under test.Accordingly, there also exists a need in the industry for an electricaltest probe that may be rotated to the desired distance without rotatingthe entire probe assembly.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to achieve a highbandwidth probe.

It is also an object of the invention to employ a probe contact springof much smaller size and therefore reduced characteristic capacitancethat also ensures good electrical contact.

It is an object of the invention to provide a test probe that may berotated to the desired distance without rotating the entire probeassembly.

The present invention achieves these and other advantageous objectives,with a high-bandwidth electrical test probe having a probe contactspring of reduced size and characteristic capacitance. The probeincludes a contact spring connected at one end to the input port of aprobe circuit. The opposite end of the contact spring enters the probesocket at a predetermined angle of entry. The probe socket has a boreformed therein which is arranged at a non-zero angle relative to theangle of entry of the contact spring into said probe socket bore,thereby guaranteeing electrical contact with the bore. The design allowsthe use of a very small contact spring, on the order of tens of mils,thereby reducing the parasitic capacitance of the spring and allowingmuch higher bandwidths than heretofore achievable.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of this invention, and many of theattendant advantages thereof, will be readily apparent as the samebecomes better understood by reference to the following detaileddescription when considered in conjunction with the accompanyingdrawings in which like reference symbols indicate the same or similarcomponents, wherein:

FIG. 1 is a block diagram of a conventional test setup;

FIG. 2 is a top view of an electrical test probe implemented inaccordance with the invention;

FIG. 3 a cross-sectional view of the electrical test probe assemblyimplemented in accordance with the invention, with the contact springpositioned not in electrical contact with the probe socket;

FIG. 4 is a cross-sectional view of the probe socket used in theelectrical test probe assembly of FIG. 3;

FIG. 5 is a cross-sectional view of the nose cone used in the electricaltest probe assembly of FIG. 3;

FIG. 6 is a cross-sectional view of the probe pin used in the electricaltest probe assembly of FIG. 3;

FIG. 7A a cross-sectional view of the electrical test probe assembly ofFIG. 3, with the contact spring positioned in electrical contact withthe probe socket;

FIG. 7B an alternative embodiment of a cross-sectional view of anelectrical test probe assembly in accordance with the invention, withthe contact spring positioned in electrical contact with the probesocket;

FIG. 8 is a coaxial view of a cross-section of the test probe assemblyof FIG. 7B;

FIG. 9 is a coaxial view of a cross-section of the test probe assemblyof FIG. 7 with the nose cone rotated 90° from the position of the nosecone in FIG. 8.

FIG. 10 is a perspective view of a prior art electrical test probe;

FIG. 11 is a side view of the prior art contact spring used in theelectrical test probe of FIG. 10; and

FIG. 12 is a top view of the prior art contact spring of FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings, FIG. 1 illustrates a test setup environment2 for measuring a signal on a test point 5 of an electronic circuitunder test 4.

The test setup environment 2 includes an electronic instrument 6 (e.g.,an oscilloscope, spectrum analyzer, or logic analyzer) connected to atest probe 10. The test probe 10 comprises a probe tip 12 which may beplace in electrical contact with the test point 5 of the circuit undertest 4. The probe tip typically comprises circuitry (internal to theprobe tip 12) for filtering, conditioning, and amplifying the signalseen on the test point prior to passing it on to the test instrumentover a probe cable 14.

FIG. 2 shows a top view of an electrical test probe 100 implemented inaccordance with the invention. As illustrated, electrical test probeassembly 101 comprises an electrical cable 102 connected to a connector105 (e.g., a BNC connector) at one end 103 and to a probe assembly 100at the opposite end 104 of the cable 102.

FIG. 3 shows a cross-sectional view of the electrical test probeassembly 101 implemented in accordance with the invention. Asillustrated therein, the electrical test probe assembly 101 comprisesprobe circuitry 124 implemented on a printed circuit board 120 forfiltering, conditioning, and amplifying a signal detected by a probe tip110, discussed hereinafter. Preferably, all such circuitry isimplemented within an integrated circuit 124 on the printed circuitboard 120. The printed circuit board 120 includes an input port 125 forreceiving signals detected by the probe tip 110, and an output port 126for electrical connection to the electrical cable 102.

The printed circuit board 120 is positioned within a housing 112. In theillustrative embodiment, the housing 112 is a cylindrical barrel 114with a coaxial bore 115 formed therein. The probe cable 102 enters oneend 117 of the cylindrical barrel 114 and is electrically connected tothe output port 126 of the printed circuit board 120. A contact spring130 is electrically connected to the input port 125 of the printedcircuit board 120, and exits the opposite end 118 of the cylindricalbarrel 114.

The barrel 114 is connected to a probe tip 110, which includes a nosecone 140, probe socket 150, and probe pin 160. The nose cone 140 isconfigured to house the probe socket 150 and probe pin 160. The contactspring 130 exiting the barrel 114 is electrically connectable to theprobe pin 160 within the probe socket 150.

In a preferred embodiment, the contact spring 130 is compressiblyconnectable to the probe socket 150 via a compression spring 180 (alsocalled a z-compliance spring) housed in the bore shaft 115 of thebarrel. Decompression tab 184 is attached to the printed circuit board120 and are slidable along the coaxial axis of the bore shaft 115 of thebarrel. The decompression tab 184 protrudes to the exterior of thebarrel 112 and slides parallel to the axis of the coaxial bore 102 ofthe barrel 112. In the fully released position, the compression springprojects the printed circuit board 120 along the coaxial axis of thehousing in the direction of the probe tip 110, exerting sufficient forceagainst the printed circuit board 120 to ensure that the contact spring130 is fully inserted in electrical contact with the probe socket 150,as described hereafter. Electrical contact between the probe pin 160 andprinted circuit board 120 may be broken by manually positioning the tab184 in the direction opposite the probe tip 110, thereby compressing thespring 180 to cause the contact spring to exit the probe socket 150 andlose electrical contact therewith.

The compression spring operates to position the contact spring 130either in or not in electrical contact with the probe pin 160 bycompressing the printed circuit board 120 in the barrel 114 either inthe direction of, or in the opposite direction of, the probe tip 110relative the barrel 114.

With further reference to FIG. 4, in the preferred embodiment, the probesocket 150 comprises a substantially cylindrical barrel structure 151with a coaxial bore 152 formed therethrough. In the preferredembodiment, the coaxial bore 152 of the probe socket 150, hereinafter“probe socket bore 152”, comprises a conical probe socket bore section153, a first equi-diameter probe socket bore section 154, and a secondequi-diameter probe socket bore section 155. The conical probe socketbore section 153 forms a bore having a diameter gradually decreasingfrom a maximum diameter opening at one end 156 of the probe socket 150to a minimum non-zero diameter opening into the first equi-diameterprobe socket bore section 154.

Preferably, in order to ensure maximum electrical contact, the diameterof the first equi-diameter probe socket bore section 154 substantiallymatches, or is only slightly greater than, the diameter of the contactspring wire 130. In the preferred embodiment, the diameter of the firstequi-diameter probe socket bore section 154 is approximately 10 mils.The first equi-diameter probe socket bore section 154 opens at one endinto the conical probe socket bore section 153 and opens at the otherend into the second equi-diameter probe socket bore section 155.

The diameter of the second equi-diameter probe socket bore section 155preferably substantially matches, or is only slightly greater than, thediameter of the plug end 164 of the probe pin 160 to ensure maximumelectrical contact between the probe socket bore section 155 and theprobe pin 160. In the preferred embodiment, the diameter of the secondequi-diameter probe socket bore section 155 is approximately 20 mils.

A probe pin plug 164 of a probe pin 160 is fitted into the secondequi-diameter probe socket bore section 155 on the socket end 157 of theprobe socket 150. In the preferred embodiment, the probe pin plug 164 isapproximately 20 mils.

At the opposite end 156 of the probe socket 150 proximate to the printedcircuit board 120 of the test probe 100, the contact spring 130 fitsthrough the conical probe socket bore section 153 and into the firstequi-diameter probe socket bore section 154. In the preferredembodiment, the diameter of the contact spring wire 130 is approximately10 mils.

Referring now to FIG. 5 in conjunction with FIGS. 3 and 4, in thepreferred embodiment, a nose cone 140 houses the probe socket 150. Anose cone bore 142 formed within the nose cone 140 comprises includes aconical nose cone bore section 143 opening into and arranged at adifferent angle relative to a first and second nose cone bore section144 and 145. The first nose cone bore section 144 substantially conformsto the exterior shape and size of the probe socket 150. Preferably, theexterior of the probe socket 150 includes a recess 158 on its exterior,and the first nose cone bore section 144 of the nose cone bore 142includes a mating tab 148 that substantially fits within the exteriorprobe socket recess 158.

The exterior probe socket recess 158 on the exterior of the probe socket150 and the mating tab 148 on the interior wall of the first nose conebore section 144 together form a snap lock. The snap lock operates tolock the probe socket 150 into place when it is inserted fully into thefirst nose cone bore section 144 of the nose cone bore 142. In thisregard, the probe socket 150 and/or mating tab 148 of the first nosecone bore section 144 is made of a sufficiently flexible material toprovide a sufficient amount of give to allow the non-recessed exteriorportion of the probe socket 150 to pass over the tab 148 as the probesocket 150 is inserted into the first nose cone bore section 144. Oncethe probe socket 150 is inserted far enough that the tab 148 passes intothe recess 158 on the probe socket 150, the probe socket 150 is lockedsecurely in place. In the preferred embodiment, the nose cone 140 isformed as a molded plastic part. The molded plastic provides sufficientflexibility to allow insertion of the probe socket 150 into the firstnose cone bore section 144 but is sufficiently inflexible such that theprobe socket 150 is not easily removable once the tab passes into therecess of the probe socket 150.

The second nose cone bore section 145 houses a probe pin 160. The probepin 160 comprises a probe pin shaft 163 with a probe pin plug 164situated at one end of the probe pin shaft 163, and a probe pin head 162situated at the opposite end of the probe pin shaft 162. The probe pinhead 162 is preferably conical in shape with a point at one end whichoperates as the electrical contact tip 161. The second nose cone boresection 145 substantially conforms to the exterior shape and size of theprobe pin shaft 163 and a substantial portion of the probe pin head 162.As described previously, the probe pin 160 comprises a probe pin plug164 at one end that fits securely into one end of the probe socket 150.Preferably, the probe pin shaft 163 and probe pin head 162 fit snuglyinto the second nose cone bore section 145 of the nose cone bore 142 tofurther assist in holding the probe pin 160 securely in place. The probepin head 162 preferably extends slightly outside of the nose cone bore142 to allow the contact tip 161 to make electrical contact with pads,nodes, or pins on the device/board under test.

One end of the first nose cone bore section 144 of the nose cone bore142 opens coaxially into the second nose cone bore section 145 of thenose cone bore 142.

The other end of the first nose cone bore section 144 of the nose conebore 142 opens into the conical nose cone bore section 143 of the nosecone bore 142 where the diameter of the conical nose cone bore section143 is the smallest. The axis of the first and second sectionscoincides, and hence are coaxial. The axis A-A′ of the conical nose conebore section 143 is arranged at an angle, θ, with respect to the coaxialaxis B-B′ of the first and second nose cone bore sections 144 and 145.In the preferred embodiment, the angle θ is an obtuse angle, where90°<θ<180°.

To assemble the probe tip 110 of the electrical test probe assembly 101,the probe socket 150 is inserted into the first nose cone bore section144 of the nose cone bore 142. In the preferred embodiment, the probesocket 150 is inserted through the conical nose cone bore section 143 ofthe nose cone bore 142 and into the first nose cone bore section 144 ofthe nose cone bore 142. The probe socket 150 is then further inserteduntil the tab 148 on the interior wall of the first nose cone boresection 144 of the nose cone bore 142 snaps into the exterior recess 158of the probe socket 150, thereby locking the probe socket 150 in placewithin the first nose cone bore section 144 of the nose cone bore 142.

The contact tip of the contact spring wire 130 is then inserted into theconical nose cone bore section 143 of the nose cone bore 142.

FIG. 7A illustrates the positioning of the contact spring 130 inaccordance with a preferred embodiment of the invention. As illustratedin the exploded portion 190 a of FIG. 7A, the contact spring enters theconical nose bore section 143 along an axis B-B′ that hits the lowerwall of the conical probe socket bore section 153. Axis B-B′ is arrangedat an angle θ with respect to the axis of the first and secondequi-diameter probe socket bore sections 154 and 155.

As shown in FIG. 7A, because of the offset angle θ between the angle ofincidence of the contact spring 130 and the axis A-A′ of the probesocket bore 142, the contact spring is guaranteed a first point ofcontact 171 at the bending point of the contact spring along the wall ofthe conical probe socket bore section 153. As the wire is furtherinserted into the nose cone 140, the contact tip of the contact springwire 130 is forced along the wall of the conical probe socket boresection 153 and into the first equi-diameter probe socket bore section154. Upon further insertion, the contact tip of the contact spring wire130 eventually hits the far wall of the first equi-diameter probe socketbore section 154, ensuring a second guaranteed point 172 of electricalcontact. Further insertion of the contact spring 130 forces the slidesthe contact spring tip further into the first probe socket bore section154.

FIG. 7B shows an alternative embodiment of the probe socket assembly 101wherein the contact spring 130 enters the first probe socket boresection 154 directly, providing a single guaranteed point of contact 173at the bending point of the contact spring wire.

It will be appreciated that the offset angle θ between the axis of theprobe socket bore 152 relative the angle of incidence of the contactspring 130 when the probe socket 150 is inserted into the probe socket150 thus guarantees electrical contact between the contact spring 130and probe pin 160. In particular, because the axes are offset, thecontact spring wire 130 must bend under insertion force in at least oneplace 171, 172, 173 in order to further insert into the firstequi-diameter section 154 of the probe socket 150.

As discussed previously, the second equi-diameter probe socket boresection 155 of the probe socket 150 substantially matches the diameterof the probe pin plug 164 such that the probe pin 160 plug fits snuglyin place and in electrical contact within the probe socket 150.Accordingly, because the contact spring 130 is guaranteed to makeelectrical contact with the probe socket 150, as discussed above, thecontact spring 130 is also guaranteed to make electrical contact withthe probe pin 160.

It will be appreciated from the above detailed description that thecontact spring 130 of the electrical test probe assembly 101 is axiallyindependent of the rotational axis of the nose cone 140. The nose cone140 may thus be rotated to any angle without requiring the contactspring 130 to also rotate. FIG. 8 shows a cross-sectional view of theelectrical test probe assembly 101 when the contact spring is insertedin the probe socket of the probe tip assembly 101. As illustrated, theprinted circuit board 120 is rotationally at 0° in this example, and thenose cone is positioned arbitrarily at a 45° offset relative to theposition of the printed circuit board 120. FIG. 9 illustrates across-sectional view of the electrical test probe assembly 101 of theinvention where the nose cone 140 has been rotated away from the 0°point by another 90°. As shown, the printed circuit board 120 remains at0° relative the 0° point, while the nose cone is now positioned at 135°relative the 0° point. To accommodate the different position of the nosecone, the contact spring 130 merely bends in a different direction. Thecontact spring 130 and printed circuit board 120 have not axiallyrotated.

Although this preferred embodiment of the present invention has beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A method for assembling an electrical test probe, comprising:connecting a first end of a contact sprint to an input port of a probecircuit; determining an angle of entry of a second end of said contactspring into a probe socket bore of a probe socket; and arranging saidprobe socket such that a coaxial axis of said probe socket bore is at anon-zero angle relative to said angle of entry of said contact spring.2. A method in accordance with claim 1, comprising: electricallycoupling a probe pin to said probe socket.
 3. A method in accordancewith claim 2, comprising: inserting said second end of said contactspring into said probe socket bore.
 4. A method in accordance with claim1, comprising: seating said probe socket within a nose cone, said nosecone forming a nose cone bore therethrough, said nose cone bore having aprobe socket section configured to securely hold said probe socket inplace.
 5. A method in accordance with claim 2, comprising: seating saidprobe socket within a nose cone, said nose cone forming a nose cone boretherethrough, said nose cone bore having a probe socket sectionconfigured to securely hold said probe socket in place and a pin sectionconfigured to securely hold said pin in electrical contact with saidprobe socket.
 6. A method in accordance with claim 3, comprising:seating said probe socket within a nose cone, said nose cone forming anose cone bore therethrough, said nose cone bore having a probe socketsection configured to securely hold said probe socket in place and a pinsection configured to securely hold said pin in electrical contact withsaid probe socket.